This invention relates generally to nuclear fuel elements. More particularly, this invention relates to fuel elements which employ a cladding tube containing a fuel pellet of fissionable material for use in a nuclear reactor.
One form of a fuel rod typically employed in nuclear reactors comprises a fuel pellet which is contained in a cladding tube. The cladding tube is typically manufactured from zirconium-alloy or various metal-alloy materials. The fuel rods, in conventional reactor configurations, are mounted to a support grid. The lower portions of the rods below the grid are disposed in regions where debris may be entrapped.
The zirconium-alloy fuel cladding has a useful life which is subject to debris fretting. The zirconium-alloy fuel rods employed in nuclear reactors are exposed to high temperature water which is typically in the range 300.degree. C. to 400.degree. C. The water is subjected to high pressures and frequently contains metal particles of stainless steel or Inconel-alloy steel which originate at reactor locations remote from the fuel rods themselves. The metallic particles tend to collect near the bottom of the fuel rods and are entrapped by the first support grid for the fuel rods. The metallic debris may be maintained in a quasi-suspensive state due to vibration and movement of the water through the reactor.
The metallic particles which form the debris may be hardened by radiation. The hardened metallic state tends to rapidly accelerate the wear or erosion of the cladding tubes of the fuel rods. The resultant tube fretting may be sufficient so as to ultimately result in penetration of the cladding tube wall, thereby resulting in failure of the cladding. Thus, the long term integrity of the fuel cladding is a direct function of resistance to debris fretting.
The hostile environment of the reactor dictates that any structural modification or enhancement to the cladding tube satisfy a number of constraints. First, any wear resistant structure must be significantly harder than the metallic debris particles to effectively resist abrasion from the particles. Any coating applied to the cladding tube must have excellent long term adhesive qualities, be fully compatible with the thermal expansion of the cladding tube and also form a strong bond with the tube. In addition, any coating must be resistant to the chemical environment in the reactor which characteristically includes hot water at a pH of approximately 7. The thickness of any coating applied to the cladding tubes must be relatively thin so that the flow of water around the fuel rods is not significantly impeded by the coating and that the coating not function as a thermal barrier. Any coating is preferably capable of application in a process which does not require heating of the cladding tube above 400.degree. C. In addition, it is also desirable that the coating be inexpensive and be suitable for mass production.
Coatings of various forms and functions have been applied to the inside surfaces of cladding tubes for nuclear reactors. For example, U.S. Pat. Ser. No. 07/211/182 assigned to the assignee of the present invention discloses a fuel element for a nuclear reactor having a zirconium-tin alloy cladding tube. A thin coating of an enriched boron-10 glass containing burnable poison particles is deposited on the inside of the cladding tube from a liquid sol-gel. The coating includes a glass binder which is applied on the inside of the zirconium-alloy cladding tube.
U.S. Pat. No. 3,625,821 discloses electroplating the inside surface of a tube with coating of a matrix metal and boron compound of, for example, nickel, iron manganese or chrome. Boron compounds such as boron nitride, titanium boride and zirconium boride are electroplated onto a Zircaloy substrate. U.S. Pat. No. 4,695,476 discloses a vapor deposition of volatized boron compounds on the inside of fuel rod cladding.